Device for dewatering sludge

ABSTRACT

There is disclosed a device for dewatering sludge such as residuals in sewage discharge processing installations. The device provides that cakes are formed of the sludge. These cakes have initially a high water content and are of non-homogenous, loose consistency. They are guided between filter bands through a preliminary filtering zone in which some of the water contained in the cakes is extracted by straining and pressure action applied thereto. The filter bands with the cakes therebetween are then successively guided over a first dewatering drum and a subsequent second dewatering drum. Pressure is applied to the filter band while being guided about the drums, thereby extracting further water from the cakes. Additionally, the cakes while traveling from the feed-in point toward and through pressure zones are deformed and also turned over thereby compacting the cakes as water is extracted therefrom. Finally, the cakes after being sufficiently dewatered and compressed are discharged.

CROSS-RELATED APPLICATION

This Application is a division of Ser. No. 477,413 filed June 7, 1974and issued as U.S. Pat. No. 4,019,431.

The invention relates to a device for dewatering sludge-type material asit is produced as a by-product or residual product in sewage dischargeprocessing or similar installations.

BACKGROUND

There are known devices for dewatering sludge of the general type abovereferred to in which cakes formed of sludge to be processed are guidedbetween two filter bands about the circumferential wall of a drum. As aresult, a certain percentage of the water contained in the cakes isremoved as the cakes are guided about the drum.

A device using this mode of dewatering sludge is described, forinstance, in German DTAS No. 1,960,787. This patent describes adewatering press in which filter bands are guided partly about thecircumference of a single drum, the sludge to be processed being carriedbetween two filter bands. It has been found that such an arrangementdoes not result in an economically acceptable dewatering efficiency.

There are also known devices for the purpose from U.S. Pat. No.2,111,720 and German Pat. No. 689,090, in which guidance of filter bandsused for causing dewatering of the sludge is similarly unfavorable withrespect to efficiency.

Tests have shown that the shortcomings of dewatering devices aspreviously described are caused primarily by an insufficient break-up ofthe sludge during the dewatering operation itself.

THE INVENTION

It is a broad object of the invention to provide a novel and improveddevice for dewatering sludge which eliminates the aforepointed outshortcomings of sludge dewatering devices as heretofore known.

It is also an object of the invention to combine a high dewateringefficiency with minimal space requirements.

According to the invention, these advantages of high dewateringefficiency and minimal space requirements are obtained by increasing theactive length of the filter bands in relation to the total length of thefilter bands which, in all dewatering devices as heretofore known, waswell below 50%. Modern demands with respect to the quality of processingsewage water discharges require much higher efficiency for environmentalreasons coupled with economically acceptable costs.

SUMMARY OF THE INVENTION

The aforepointed out objects, features and advantages, and otherobjects, features and advantages which will be pointed out hereinafterare obtained by providing at least two drums as dewatering drums andguiding filter bands between which the sludge to be dewatered iscarried, at least over part of the circumferential walls of these drums.In this connection it is advantageous to drive the two drums in oppositedirections and to guide the filter bands over the circumferential wallsof the drum so that they define a generally S-shaped configuration inupright position.

The circumferential walls of the drums may be made either of corrugatedor perforated sheet metal such as steel, or they may be composed oftubes extending lengthwise of the axes of the drums. The latterarrangement assures that the filtrate as it is extracted from passingsludge will be discharged into the interior of the drums from which itcan be conveniently removed.

According to an aspect of the invention, the dewatering drums definewithin the range of their circumferential walls about which the filterbands are guided several pressure zones within which an S-shapedguidance of the filter bands causes pulling, kneading and deformingaction upon the generally cake-shaped sludge portions placed between thefilter bands. As a result, the internal consistency of the cake isloosened thereby facilitating extraction of water therefrom.

In this connection, it may be mentioned that the concept of theinvention is not limited to two drums, but in some instances it may bepreferable to provide more than two drums in side-by-side arrangementand to guide the filter bands accordingly over the circumferential wallsof the drums.

According to another aspect of the invention, sludge in the form ofsludge cakes of non-homogenous consistency, after having been guidedthrough one or more straining and/or preliminary dewatering zones (n)supported on a filter band, is guided to a further filter band whilebeing simultaneously mixed, if desired, assisted by gravitational force.Thereupon the sludge cakes are guided between the two filter bands whichare disposed in parallel relationship and preferably along a linear paththrough a preliminary pressure zone. At the end of this zone the sludgeis guided over the first one of the aforesaid dewatering drums. At thesame time, the sludge cakes may be subjected to vibratory pressure andcontinuing shear forces. After having been subjected to the action ofthis composite force resulting in a thorough loosening of the sludgecakes, the filter bands and thus also the sludge cakes therebetween aredeflected after leaving the first dewatering drum into the oppositedirection and then guided upon the second dewatering drum, that is, thesludge cakes are now subjected to a pressure zone curved in oppositionto the curvature on the first drum since, as previously stated, the twodrums being driven in opposite direction.

The aforesaid pulsing or vibrating pressure is obtained by guiding thefilter bands and thus also the sludge cakes between pressure rolls andthe circumferential wall of the dewatering drum. The just-describedaspect of the invention may also provide that the pressure force appliedto the sludge cakes via the filter bands, and more specifically by theouter filter band, is increasing in the driving direction of the bands.

According to the invention, it is also particularly advantageous thatthe envelopment angle of the filter bands on the drums which obviouslycontrols the length of the pressure zones is larger than 180°. It isfurther advantageous that the pressure or planet rolls be provided atleast along the circumferential range of the drum as determined by theafore-mentioned angle of more than 180°.

As it is evident, the longer the pressure zone is, the more intensive isthe action of the shear forces upon the sludge cake as these shearforces are generated when and while the filter bands envelope thecircumferential walls of the dewatering drums. Obviously, the outerfilter band must travel a longer distance than the inner filter band asthey are driven by the rotation of the drums. Accordingly, assumingequal driving speed for both bands, the outer band will be retardedrelative to the inner band by a distance which is equal to the productof the radial differential, that is, the radial thickness of the cakesbetween the two bands times the angle of envelopment. This retarding ofthe outer filter band causes displacement or deformations within thecakes. As a result there is a change in the consistency of the cakes,and particularly a loosening thereof, so that liquid can be more readilydriven out of the cakes. Moreover, the already described reversal of thedirection of stresses due to the opposite rotational directionsmaterially increases the dewatering effect.

It is also within the concept of the invention that the pressure of theplanet rolls, both for individual rolls and for a group thereof, can becontrolled. Such control can be effected in a particularly simple mannerby means of a tension member such as a rope. This tension member is incontact with lever arms provided according to the invention, which inturn are linked to a stand or frame structure for the drums or toanother suitable support.

The rope or other tensioning member which is tensioned along thecircumferential outline of the dewatering drums may coact for examplewith adjustable noses or dogs on the lever arms or other pivotalelements can be used. By tensioning of the rope the component of forcecomposed of the force of the rope in a direction normal to the leverarms is enlarged. As a result, the pressure exerted by the planet rollor rolls toward the respective dewatering drum and thus the filter bandsincreases corresponding to the effective length of each lever armbetween its pivot point or the setting of the nose or dog.

According to still another aspect of the invention, the pressure orplanet rolls which are preferably staggered relative to each other inthe driving direction of the filter bands, can be increased by severalpressure plates overlying the rolls and also made more uniform. Suchpressure plates can be provided on both sides of the filter bands whichare disposed substantially parallel to each other within the preliminarypressure zones, and if desirable, the pressure plates are so mountedthat they can be jointly pressed toward the bands whereby additionalcontrol of the effective pressure exerted by the rolls is obtained.

Advantageously, the pressure rolls can also be in the form of parts ofcontinuously rotating pressure bands, etc. These parts may be coupled bychain links. In actual practice, it depends on the specific conditionsunder which a particular device is operated whether the pressure bandsare directly driven or driven indirectly by being placed in drivingcontact with the filter bands.

It is also possible to arrange pressure bands or similar members so thatthey in turn will drive the filter bands without special drive for thefilter bands. The control of the bands can be effected in an efficientmanner by means of deflection rolls, or as it is described in thepresent invention, by means of the dewatering drums.

According to a particularly favorable embodiment of the device accordingto the invention, one of the filter bands is extended to the location ofa feed chute or hopper, and it extends from the point of feed abouthorizontally to a deflection roll. This distance between the feed pointand the deflection roll may be utilized as a straining or pre-dewateringzone. After passing the deflection roll, the aforementioned extendedfilter band now jointly with the second filter band which is only intangential relationship with the deflection roll, is thus moving inopposition to its initial direction of movement through the preliminarypressure zones. At the end of the movement in this opposite direction,both filter bands are jointly guided upon the first one of thedewatering drums. This drum is enveloped by the two filter bands with anenvelopment angle which according to the invention is between 220° to240°.

After having passed about the first dewatering drum, the two filterbands are guided to a further dewatering drum which is disposedside-by-side and approximately horizontally of the first drum. Theenvelopment angle of the filter bands about the second drum ispreferably larger than the envelopment angle for the first drum.

After leaving the second dewatering drum, or if there are more than twodrums after leaving the last drum, the now fully processed or workedsludge cake is discharged and each filter band is separately guided backto the straining and/or preliminary pressure zone. While moving towardthese zones, the filter bands are preferably guided into contact with atleast some of the pressure rolls on the sides thereof distant from therespective drum thus increasing the effective pressure exerted by theserolls.

A dewatering device according to the invention with two dewatering drumshaving a drum diameter of 1 meter provide about 16 meters of active bandlength and the total length of the device is not more than 2.5 meters.The thus obtained space utilization ration is more than 70%, and this isvery much in excess over the utilization ration as can be obtained withdewatering devices as heretofore known.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawing, several embodiments of the invention areshown by way of illustration and not by way of limitation.

In the drawing:

FIG. 1 is a diagrammatic elevational section of a dewatering deviceaccording to the invention including two substantially side-by-sidedisposed dewatering drums;

FIG. 2 is a detail view of FIG. 1 on an enlarged scale;

FIG. 3 is a view of a modified detail of the device according to FIG. 1on an enlarged scale;

FIG. 4 is an elevational sectional view of a modification of adewatering device according to the invention;

FIG. 5 is a detail view of FIG. 4 on an enlarged scale and

FIG. 5A is a detail view of FIG. 5 on a further increased scale.

DETAILED DESCRIPTION OF THE DRAWING

Referring now to the figures in greater detail, and first to FIG. 1,this figure shows an exemplification of a dewatering device R whichincludes two large dewatering drums 1 and 2. Two filter bands 4 and 5which are disposed for part of their length in parallel spaced apartrelationship, envelop the two drums for part of the circumferentialwalls thereof. More specifically, the envelopment angle for drum 1 is m(for instance 235°), and n (for instance 285°), whereby the two filterbands approximately define an upright S-shape.

A cake Q formed of sludge to be processed is fed in the direction t uponfirst filter band 4 in the conveying direction i. This band guides thecakes Q by means of support rolls 6 through straining zone D₁ to effectpreliminary dewatering of the cakes. Water extracted from the cakes isremoved through the mesh openings or holes in the pulling side 4_(o) ofband 4 and drops into a catch trough 7.

Each preliminarily dewatered cake Q falls at a deflection roll B forfilter band 4 upon the pulling side 5_(o) of filter band 5 which isdisposed approximately tangentially with respect to the lower side ofdeflection roll 8. As a result, the cakes are successively turned aroundand also mixed. Between the idling side 4_(u) of the first filter band 4and the pulling side 5_(o) of the second filter band 5 the sludge cakesare guided through a preliminary press zone D₃. In this zone pressurerolls 9 which are disposed staggered on both sides of filter band sides4_(u) and 5_(o) process the sludge cakes. Both rows of pressure rolls 9are pressed via pressure plates P against each band side 4_(u) and 5_(o)by means of power sources 10 coacting with pressure plates P.

The deformation such as bends introduced by pressure rolls 9 in thefilter bands are ignored in the drawing to simplify the illustration.

After leaving the preliminary pressure zone D₃ the two filter bands 4and 5 with sludge cakes Q therebetween are guided upon thecircumferential wall of dewatering drum 1 in the rotational direction i,within the range of the envelopment angle m. This angle controls theeffective length of the main pressure zone D₅ in which the filter bands4 and 5 are guided between the circumferential wall 11 of the drum onone hand and planet rolls 40 disposed in juxtaposition to the outer sideof the circumferential wall of the drum.

The drum wall 11 consists either of grooved or perforated metal sheetssuch as steel sheets, or according to FIG. 2, out of tubes 41 which aredisposed parallel to the rotational axis of the drum. The filtrate whichis pressed out of the cakes flows between the tubes into the interior Jof the drum. The tubes 41 are bored at their ends (not shown) so thatany filtrate accumulating in the lower part of the space U within thedrum flows first into the space within the tubes and can then be removedfrom the tubes at the ends thereof.

The planet or pressure rolls 40 are biased by the action of powersupplies 42 toward the outside wall 11 of the drum. Such bias may beobtained either for each one of the planet rolls 40 or for groups ofthese rolls. Each roll 40 is pivotally supported by means of a link 44hinged to a pivot 46 as schematically indicated by supports 45 orsimilar means. These pivotal links 44 mount dogs or noses 47 for placingthereupon a pull rope 48. The pulling force exerted by this rope can beadjusted by the power supply 42 shown as a rotary pulley.

The aforedescribed pressure means (42 to 48) permit by utilization ofthe leverage to effect individual regulation of the pressure for eachone of the planet rolls. By tensioning rope 48 the component of force asproduced by the rope pull in a direction normal to the pivotal link 44is increased, whereas the pressure of the planet roll 40 toward drumwall 11 varies the respective lever arm between the pivot point 46 oflink 44 and the respective nose for supporting the pull rope 48.

The pressure of the planet rolls toward the respective drum wall isfurther enlarged by the tension of the returning filter band portions4_(e) and 5_(e) within each range C in which the planet rolls 40 in totoor individual planet rolls 40_(z) are utilized for effecting thedeflecting of the filter bands. The point at which the filter bands 4and 5 leave drum 1 and thus the pressure zone D₅ is controlled by thelower leg of the envelopment angle m.

At this stage of processing each sludge cake Q which was bent to acurvature somewhat less than coresponding to the radius r of dewateringdrum 1, is suddenly flattened between filter bands 4, 5. As a result,the previously inner parts of the cake are stretched and the previouslyoutside lying cake parts are pushed together whereby the structuralconsistency of the cake is correspondingly loosened and prepared forfurther pressing operations.

As is now apparent, the action upon the sludge cakes is similar to theone to which the cakes have been subjected in the straining zone D₁ andthe preliminary pressure zone D₃ due to the action of the gravitationalforce.

When the outer wall surface 11 of the second dewatering drum 2 isreached at the end of the comparatively pressure-free zone F the sludgecakes Q which are now loosened as described are again bent but now inthe opposite direction i₃. As a result, the effect due to thesimultaneous action of the pressure due to the tension of the filterbands and due to the first planet rolls 40_(a), is further considerablyincreased. Due to such preparation, it is possible to extract from thesludge cakes, which have now a totally different structural consistency,considerable quantities of liquid as they are guided about thedewatering drum 2. Such further extraction of water would not bepossible without the aforedescribed relocation of the sludge particlesforming the cake. The continuous shear action as it is obtained by themore rapidly moving inner filter band 4 further substantially assiststhe dewatering operation.

The dewatering drums 1 and 2 can be driven with synchronous speed orwith at least a minimum differential of circumferential speed. For thispurpose, a power drive with gearings which have different transmissionratios as hereinafter described can be used.

Filter bands 4, 5, after leaving dewatering drum 2 and the range ofrolls 40_(Z) together with the dewatered sludge cakes now designated byO are guided over a diverter roll 150 common to both filter bands tosmall individual rolls 52. The roll 150 is disposed at the apex of animaginary triangle. This triangle is defined by a line L which extendsthrough the axes of the drums, as it is shown in FIG. 1 and legsdefining an angle v of about 35°. The roll 150 is associated with planetrolls 51; in addition, some of the planet rolls 40 associated with theadjacent dewatering drum 1 touch part of the filter band sides 4_(e) and5_(e).

The two individual rolls 52 are mounted spaced apart from each other anddefine a discharge gap 53 between filter bands 4, 5 for ejectingdewatered sludge cakes O. The return or pulling band portions 4_(e) and5_(e) respectively, extend from discharge gap 53 first over the planetrolls 40 (range C), then via the upper deflector rolls 55, back to thestraining zone D₁ and to the preliminary pressure zones D₃,respectively. Band flushing means 56 are interposed between thedeflector rolls 55 and zones D₁ and D₃.

FIG. 3 shows another exemplification of the preliminary pressure zonesD₃. According to this exemplification, pressure rolls 9_(u) are disposedunderneath band sides 4_(u) and 5_(o) and also rolls 9_(o) over whichvia roller bearings 60, engaged by chain links 61 are joined to formendless pulling band portions 62_(o) and 62_(u) in the direction ofarrow S. Accordingly, a structure is formed which is similar to theflexible cage of, for instance, a needle bearing.

Driving of each pressure roll 9 is effected by frictional driving offilter bands 4, 5. The pressure rolls 9 are pressed within the pressurezones D₃ upon the inner side of filter bands 4 and 5. Diametricallyopposite to pressure rolls 9 there is disposed a pressure plate P₁ andP₂, respectively. When now the chain links 61 are guided over thesprocket teeth 63, 64 in the driving direction i of the filter bands 4and 5, the pressure rolls 9 are rolling off on the stationary pressureplates P.

The rotational speed on the opposite side is now twice as high as therunning speed of the chains formed by links 61. This has the advantagethat by means of relatively thin-walled and light pressure rolls 9, highpressures can be transmitted. This is due to the fact that there is nostrain due to the absence of bending forces normal to the axis of therolls. The bending forces are compensated by pressure plates P overlyingthe rolls.

The pressure plates P are spring tensioned at 65 so that no forces needto be transmitted by an external support structure.

In particular, if the bands are fairly light, the sprockets 63, 64 forthe chain links can be driven and then in turn drive the filter bands 4and 5 in the direction i via the pulling band sides 62, that is, thedrive of filter bands 4 and 5 is effected no longer by one of thedeflection rolls 8, 5_(o), 55 or the dewatering drums 1 and 2 whichwould cause heavy strains within the range of the transmission of forceupon the respective filter band 4 or 5, which must be pulled by amultitude of gaps between rolls. Instead of this transmission of forcethere is obtained at each of the gaps between the rolls a transmissionof force corresponding to the prevailing pressure, and this transmissionof force is uniformly distributed over a large surface.

By selecting different diameters d and axial spacings f for the upperand lower pressure rollers 9, substantially more or less pronouncedsine-shaped deflectors can be obtained at the surfaces of the filterbands between which sludge cakes Q are placed. Moreover, relativemovement can be superimposed by differential speeds of the upper andlower pulling sides 62_(o) and 62_(u) of the bands.

According to the exemplification shown in FIG. 4, the device R₁ does notinclude the straining zone D₁, but the sludge cakes Q drop immediatelyinto preliminary pressure zone D₃ which is disposed below dewateringdrums 1 and 2. Pressure rolls 9 are loaded by means of pressure platesP. These plates in turn are swingingly supported on catch trough 7.

The sludge cakes Q are guided from the preliminary pressure zone D₃ tothe first one of the dewatering drums. After leaving this drum, cakes Qare guided through pressure zone D₅ and subsequently to pressure zoneD₇. After having passed through the last-mentioned zone, filter band 4guides the now dewatered cake O to deflection roll 90 which is continuedby a discharge chute 91. Filter band 4 after having been guided overroll 90 passes through cleaning zone 56 and a subsequent catch trough93. This catch trough is formed by two rolls 92 on band 4 itself.Finally, band 4 is guided by deflection rolls 55 and 8 back topreliminary pressure zone D₃.

The second filter band 5 is guided over and between the dewatering drums1 and 2 by the lower band 4 and it is also lifted by this band above thedrums and either immediately at planet rolls 40 or over furtherdeflection rolls and cleaning station 56 back to preliminary pressurezone D₃. Cleaning station 56 includes discharge elements 94 whichaccumulate the cleaning water and discharge the same via pipes 95.

If it is desired to effect preliminary dewatering also with theexemplification R₁, filters may be provided ahead of the feed-in points(not shown).

Referring now to FIG. 5, according to this figure a further belt 3formed by filter bands 4 and 5 is guided over dewatering drum 1 in thedriving direction i. Sludge cakes Q are again placed between the filterbands. The two filter bands 4 and 5 are passed about the circumferentialwall surface 11 of drum 1 within an enveloping angle m of about 235°,said angle being the sum total of the so-called roller angles m₁. Thefilter bands leave dewatering drum 1 which rotates in the direction i₁at the lower branch 50 of enveloping angle m.

A plurality of planet rolls 40 press the outer or upper filter band 4toward the circumferential surface wall of drum 1 within pressure zonesD₅ which in turn is controlled by the envelopment angle m.

The axle 70 of each planet or pressure roll 40 is journalled at bothends by radially disposed bars 71. The end of these bars which faces thedrum shaft F mounts a lock nut 72. A coil spring 74 which is positionedto expand in radial direction is disposed between nut 72 and abutmentplates 73 which in turn are supported by diagrammatically indicated drumholders 45. Each of these springs biases the respective pressure roll 40toward the circumferential wall 11 of the drum.

The axles 70 of the rolls are staggered in circumferential direction bypull links 75. The axle 70_(a) which is the first one in the drivingdirection i is suspended by means of radially disposed guide bars 76 ona cross bar 77 on the drum support 45.

The effective length c of the guide parts 76 can be adjusted by settingan adjustment nut 78. Similarly, the effective length of radiallydisposed bars 71 can be adjusted by means of nuts 72.

A dewatering device as exemplified, that is, a device which includes twodewatering drums 1 and 2, provides that with a drum diameter of 1 meterabout 16 meters of effective filter band length are available. Yet thetotal length of the device is not more than 2.5 meters. The resultingutilization of the filter bands is more than 70%, and such highefficiency factor is far above the efficiency factor of any dewateringdevice of the kind herein-referred to as now known.

While the invention has been described in detail with respect to certainnow preferred examples and embodiments of the invention, it will beunderstood by those skilled in the art, after understanding theinvention, that various changes and modifications may be made withoutdeparting from the spirit and scope of the invention, and it isintended, therefore, to cover all such changes and modifications in theappended claims.

What is claimed is:
 1. A device for dewatering sludge in the form ofcakes, said device comprising in combination:a first rotary dewateringdrum; a second rotary dewatering drum disposed side-by-side with saidfirst drum, said drums being rotatable in opposite directions; twoparallel spaced apart filter bands for supporting and conveying sludgecakes placed therebetween, said filter bands being guided around part ofthe circumferential wall of the first drum for conveying the bands inthe rotational direction of the first drum and also being guided aroundpart of the circumferential wall of the second drum for conveying thebands in the rotational direction of the second drum and thus inopposition to the moving direction of the bands while being conveyedaround the first drum; said filter bands being guided around and by saiddrums to define an apporximately S-shape in an upright plane; saidcircumferential walls of the drums being water permeable; pressure rollsdisposed at the walls of the drums for pressing the filter bands againstthe circumferential wall of at least one of said drums; said filterbands being guided to constitute a straining and preliminary pressurezone for causing partial dewatering of sludge cakes upstream of saidfirst rotary drum, and guide rolls at the inlet end of said strainingand preliminary pressure zone for guiding the filter bands into saidzone.
 2. The device according to claim 1 wherein the angle ofenvelopment through which said filter bands are guided by and aroundsaid drums is more than 180° for at least one drum.
 3. The deviceaccording to claim 1 and comprising further guide rolls guiding saidfilter bands along a substantially linear path while constituting saidstraining and preliminary pressure zone.
 4. The device according toclaim 1 wherein one of said filter bands is extended past the otherfilter band, said extended filter band portion constituting a feedstation for feeding sludge cakes to the device, and comprisingpreliminary dewatering means for removing water from cakes, saidextended filter band portion being guided past said preliminarydewatering means for coaction therewith, deflector means for deflectingsaid extended filter band portion into said parallel relationship withthe other filter band, said filter bands being guided through saidpreliminary pressure zone prior to being guided to said rotary drums. 5.The device according to claim 4 wherein said preliminary pressure zoneincludes substantially linearly disposed pressure means.
 6. The deviceaccording to claim 4 wherein the filter band having said extendedportion is guided over part of the circumferential wall of the firstrotary drum prior to feeding sludge cakes upon said extension.
 7. Thedevice according to claim 1 wherein one of said filter bands is guidedparallel to the other filter band while passing through said preliminarypressure zone and also while being guided over part of thecircumferential walls of said drums, said one filter band beingdeflected away from the other filter band after being guided over saidwall part of the second drum to define a discharge opening for dewateredsludge cakes.
 8. The device according to claim 7 wherein deflectionmeans for deflecting said one filter band away from the other filterband are provided between said two drums.
 9. The device according toclaim 7 wherein said filter bands are guided around the circumferentialwall of the second drum through an angle of at least 270°.
 10. Thedevice according to claim 1 wherein said pressure rolls in conjunctionwith the circumferential wall of the respective rotary drum constitutesa pressure zone.
 11. The device according to claim 1 comprising settingmeans for individually varying the pressure of each of said pressurerolls toward the wall of the respective drum.
 12. The device accordingto claim 1 and comprising means ganging a plurality of said pressurerolls for joint action, and setting means coacting with said gangingmeans for varying the pressure of each plurality of ganged pressurerolls toward the wall of the respective drum.
 13. The device accordingto claim 1 and comprising adjustment means for varying the positions ofthe pressure rolls relative to the wall of the respective drum therebycorrespondingly varying the pressure applied by said rolls upon thefilter bands and thus upon sludge cakes between the filter bands. 14.The device according to claim 13 wherein said adjustment means compriselever arms each mounting at one end a pressure roll, the other end ofeach lever arm being hinged to a suspension means.
 15. The deviceaccording to claim 14 wherein said suspension means comprise a commoncarrier for a plurality of said lever arms, and comprising yieldabletension means biasing said lever arms toward the wall of the respectivedrum, and setting means coacting with said tensioning means for varyingthe bias force thereof.
 16. The device according to claim 1 andcomprising connection means interconnecting said pressure rolls inpositions substantially parallel to the circumferential wall of therespective drum, and also connecting the interconnected rolls to astationary point.
 17. The device according to claim 16 wherein saidconnection means comprise strips extending between each two adjacentpressure rolls and being attached to the axles of the respective rolls.18. The device according to claim 16 wherein said stationary point islocated at the first pressure roll as seen in the rotational directionof the respective drum.
 19. The device according to claim 16 whereinsaid pressure rolls are interconnected in the circumferential directionof the drum and also with a common stationary suspension point, and saidbias means are coupled to the pressure rolls to apply a radiallydirected force on the rolls.
 20. The device according to claim 1 andcomprising preliminary pressure means in said straining and preliminarypressure zone including further pressure rolls disposed on oppositesides of said filter bands staggered in the direction in which saidfilter bands are driven.
 21. The device according to claim 20 whereinpulling bands connect said pressure rolls on one side of said filterbands.
 22. The device according to claim 1 and comprising drive meansdriving said dewatering drums at different circumferential speeds.